The present application claims priority to Japanese Application No. P2000-139152 filed May 11, 2000, which application is incorporated herein by reference to the extent permitted by law.
This invention relates to a positive electrode active material containing a lithium transition metal composite oxide, a non-aqueous electrolyte cell employing the positive electrode active material, and a method for the preparation thereof.
In keeping up with the recent tendency towards a smaller size and a cordless structure of a variety of electronic equipment, there is raised a strong demand for a higher capacity and a lighter weight of a secondary cell as a driving power source therefor. Since a higher capacity can be achieved with a lithium secondary cell than is possible with a conventional secondary cell, a variety of proposals have been made for a cell which exploits the doping/undoping of lithium ions.
Among the positive electrode active materials for the cell, there are known lithium transition metal composite oxides, such as LiCoO2, LiNiO2, LiMnO4 or LiFePO4. Of these, LiCoO2 is preferentially put to practical use because it has a layered structure which lends itself to diffusion of lithium ions and also because it has a stable structure against doping/undoping of lithium ions. Although LiNiO2 has a layered structure and has merits such as low cost and high capacity, it is subjected to phase changes during lithium ion doping/dedoping which are more severe than those occurring in LiCoO2, such that, when LiNiO2 is used as a positive electrode active material for a lithium ion cell, the layered structure is collapsed with the progress of the charging/discharging cycles, thus leading to the lowered capacity.
As a method for overcoming the above drawback inherent in LiNiO2, such a method has been proposed in which a hetero element is substituted for part of nickel. This method may be exemplified by a method of substituting cobalt or manganese for part of nickel, for stabilizing the layered structure, as disclosed for example in Japanese Patent Laid Open Hei-5-325966 or in Japanese Patent Laid Open Hei-5-299092, and a method of substituting sodium or potassium for part of lithium, for maintaining a lithium layer which has become labile due to lithium doping/dedoping, as disclosed for example in Japanese Patent Laid Open Hei-6-150929.
There has also been researched a method for substituting plural elements for nickel. For example, reports have been made of a method of substituting plural elements selected from the group consisting of cobalt, aluminum, boron and magnesium for nickel to improve charging/discharging cyclic characteristics, as disclosed in Japanese Patent Laid Open Hei-9-92285, and of a method of substituting transition metal elements, such as magnesium, boron or gallium for nickel for improving cyclic characteristics, rate characteristics or cell reliability, as disclosed in Japanese Patent Laid Open Hei-10-208744.
However, since LiCoO2 inherently has a labile layered structure, the lithium ion cell, employing LiCoO2, has cyclic characteristics more favorable than if LiNiO2 is used. So, LiCoO2 has been practically used without the necessity of substitution by hetero atoms. However, with the recent tendency towards higher density and higher temperature of the integrated circuit, the temperature in the equipment is becoming higher, while resistance against environmental factors, required of portable equipment, are becoming stringent. So, for enlarging the usage of a lithium ion cell, it is necessary to improve the resistance against environmental factors of the cell, such as cyclic characteristics under elevated temperatures.
As for substitution of hetero elements in LiCoO2, reports have been made of substituting aluminum, indium or tin for cobalt for improving electronic conductivity, as disclosed in Japanese Patent Laid Open Sho-62-124707, or of substituting boron, bismuth or lead for cobalt, as disclosed in Japanese Patent Laid Open Hei-4-253162.
However, we have found that, when aluminum, indium or tin is substituted for cobalt in LiCoO2, the charging/discharging efficiency or capacity is lowered appreciably, and that, when boron, bismuth or lead is substituted for cobalt, high-temperature storage characteristics are lowered appreciably. So, this substitution cannot but be said to be practically undesirable. On the other hand, there is a statement in the Japanese Patent Laid Open Hei-9-92285 to the effect that, if aluminum or magnesium is substituted for part of LiCoO2, the cyclic characteristics are lowered without being improved.
It is therefore an object of the present invention to provide a positive electrode active material having improved cyclic characteristics under elevated temperatures, a non-aqueous electrolyte cell employing the positive electrode active material, and a method for the preparation thereof.
In one aspect, the present invention provides a positive electrode active material containing a lithium transition metal composite oxide represented by the general formula LiCoxAyBzO2 where A denotes at least one selected from the group consisting of Al, Cr, V, Mn and Fe, B denotes at least one selected from the group consisting of Mg and Ca and x, y and z are such that 0.9xe2x89xa6x less than 1, 0.001xe2x89xa6yxe2x89xa60.05 and 0.001xe2x89xa6zxe2x89xa60.05.
In the positive electrode active material, according to the present invention, which uses a compound as a solid solution of a combination of a first group-consisting of Al, Cr, V, Mn and Fe and a second group consisting of Mg and Ca is substituted for part of Co, not only the stability at ambient temperature but also that at elevated temperatures may be improved.
In another aspect, the present invention provides a non-aqueous electrolyte secondary cell including a positive electrode, a negative electrode and a non-aqueous electrolyte interposed between positive and negative electrodes, in which the positive electrode uses a positive electrode active material containing a lithium transition metal composite oxide represented by the general formula LiCoxAyBzO2 where A denotes at least one selected from the group consisting of Al, Cr, V, Mn and Fe, B denotes at least one selected from the group consisting of Mg and Ca and x, y and z are such that 0.9xe2x89xa6x less than 1, 0.001xe2x89xa6yxe2x89xa60.05 and 0.001xe2x89xa6zxe2x89xa60.05.
In the non-aqueous electrolyte cell according to the present invention, in which the positive electrode uses a positive electrode active material in which a combination of a first group consisting of Al, Cr, V, Mn and Fe and a second group consisting of Mg and Ca are substituted for part of Co, not only the stability at ambient temperature but also that at elevated temperatures may be improved. Moreover, the non-aqueous electrolyte cell employing this positive electrode active material is superior in charging/discharging characteristics at elevated temperatures.
In another aspect, the present invention provides a method for the preparation of a positive electrode active material including the steps of mixing a cobalt compound, a lithium compound, a compound of at least one element selected from the group consisting of aluminum, chromium, vanadium, manganese and iron and a compound of at least one element selected from the group consisting of magnesium and calcium, at a pre-set ratio, and sintering a mixture from the mixing step to produce a compound represented by the general formula LiCoxAyBzO2 where A denotes at least one selected from the group consisting of Al, Cr, V, Mn and Fe, B denotes at least one selected from the group consisting of Mg and Ca and x, y and z are such that 0.9xe2x89xa6x less than 1, 0.001xe2x89xa6yxe2x89xa60.05 and 0.001xe2x89xa6zxe2x89xa60.05. The compound of at least one element selected from the group selected from the group consisting of magnesium and calcium, as used in the mixing step, is magnesium carbonate or calcium carbonate.
In the method for the preparation of a positive electrode active material, according to the present invention, in which magnesium carbonate or calcium carbonate is used as the magnesium or calcium compound, the positive electrode active material produced is superior in stability.
In another aspect, the present invention provides a method for the preparation of a non-aqueous electrolyte secondary cell including a positive electrode, a negative electrode and a non-aqueous electrolyte interposed between the positive and negative electrodes, including, in producing positive electrode, the steps of mixing a cobalt compound, a lithium compound, a compound of at least one element selected from the group consisting of aluminum, chromium, vanadium, manganese and iron and a compound of at least one element selected from the group consisting of magnesium and calcium, at a pre-set ratio, and sintering a mixture from the mixing step to produce a compound represented by the general formula LiCoxAyBzO2, where A denotes at least one selected from the group consisting of Al, Cr, V, Mn and Fe, B denotes at least one selected from the group consisting of Mg and Ca and x, y and z are such that 0.9xe2x89xa6x less than 1, 0.001xe2x89xa6yxe2x89xa60.05 and 0.001xe2x89xa6zxe2x89xa60.05. The compound of at least one element selected from the group consisting of magnesian and calcium, as used in mixing step, is magnesium carbonate or calcium carbonate.
In the method for the preparation of a non-aqueous electrolyte cell, according to the present invention, in which magnesium carbonate or calcium carbonate is used in producing the positive electrode active material as the magnesium or calcium compound, the positive electrode active material is superior in stability, so that the non-aqueous electrolyte cell produced is superior in high-temperature charging/discharging characteristics.
According to the present invention, a lithium transition metal composite oxide represented by the general formula LiCoxAyBzO2 where A denotes at least one selected from the group consisting of Al, Cr, V, Mn and Fe, B denotes at least one selected from the group consisting of Mg and Ca and x, y and z are such that 0.9xe2x89xa6x less than 1, 0.001 xe2x89xa6yxe2x89xa60.05 and 0.001xe2x89xa6zxe2x89xa60.05, is used as the positive electrode active material, so that a non-aqueous electrolyte cell may be provided in which the positive electrode active material is improved in stability not only at ambient temperature but also at higher temperature and which is superior in high-temperature charging/discharging characteristics.